46 COMPARATIVE ANATOMY 



before any albumen is deposited around the ovum. If fertilization has 

 occurred, the ''eggs" which are "laid" contain not ova but embryos at 

 early stages of development. 



Modern mammals, with two exceptions, are viviparous. The excep- 

 tions are Ornithorhynchus and Echidna, mammals of obviously primitive 

 t3^e and reptilian in many features. The large egg covered by a thick 

 tough shell makes necessary internal fertilization, as in reptiles. All 

 other mammals, by reason of their viviparity, require internal fertiliza- 

 tion. The copulatory apparatus becomes most highly specialized in 

 mammals. 



PROTECTION, NUTRITION, AND RESPIRATION, DURING DEVELOPMENT 



From the beginning of the development of the animal the building up 

 of new protoplasm and the elaboration of it into the organized parts of 

 the animal calls for food to serve as building material. The energy 

 expended in the constructive processes of development is derived from the 

 chemical breaking down and oxidation of organic materials. Therefore 

 food and oxygen are requisite throughout development. The chance that 

 the young animal will attain adulthood depends on the degree of immunity 

 from unfavorable conditions and inimical agencies in the environment. 

 A survey of the whole group of vertebrates reveals the most extraordinary 

 diversity as regards provision for protection, nutrition and respiration 

 during development. Comparing various vertebrate embryos at corre- 

 sponding stages, the most striking differences are those which relate to 

 protection, food and oxygen. The visible differences which identify one 

 embryo as potentially a fish and another a mammal are insignificant in 

 contrast to the spectacular difference occasioned by the elaborate but 

 merely temporary placental structures of the mammal embryo. 



In Fishes. In the majority of fishes the eggs, fertilized after they are 

 discharged into the water, are exposed to the hazards of the environment. 

 Mortality is high. The eggs of some fishes (cod, mackerel, haddock, flat- 

 fishes and others) are of such low specific gravity that they float at or 

 near the top of the water ("pelagic" eggs). Others (herring, salmon, 

 trout, most fresh-water fishes, some shallow-water marine fishes) lie on 

 the bottom ("demersal" eggs) until the young hatch. Pelagic eggs do 

 not receive parental protection. Demersal eggs commonly have an 

 investing membrane which is adhesive so that the eggs stick together in 

 masses or may become attached to stones, shells, aquatic plants or other 

 submerged objects. 



Many fishes, especially those of fresh water, arrange nests of a simple 

 sort. The eggs may be deposited in a hollow scooped out in gravel, sand or 

 mud. The female salmon covers her eggs with gravel. An Australian cat- 

 fish places the eggs in a hole dug out in the sand and then covers them with 



